Various methods for the preparation of highly dispersed metals in a zero valence state have been established and widely used for the preparation of catalysts. One of the most common is the impregnation of a metal salt onto a support followed by high temperature reduction to the metallic state. However, the temperatures required for reduction often lead to sintering of the metal and can sometimes degrade the support.
Reduced metals can also be electrodeposited onto supports. U.S. Pat. No. 4,115,322 describes such a process whereby a thin film of a noble metal or alloy thereof, or silver, or an alloy of noble metals and silver or nickel is electrodeposited on a nonporous pyrolytic graphite surface. The process employs a salt of the noble metal and a complexing agent. The materials which result are electrochemical catalysts said to be useful in processes including oxygen adsorption, oxygen and hydrogen evolution, oxide formation and oxygen reduction.
Another U.S. Pat. No. 4,411,742, describes the electrolytic deposition of zinc and graphite onto metal items such as fasteners to impart a corrosion resistant surface having a low coefficient of friction. The codeposit is applied by immersing the metal substrate in an acidic zinc plating bath containing insolubilized bulk graphite.
U.S. Pat. No. 3,703,446 relates to fluidized bed electrolytic cells which can produce homogeneous metal powders and powders with layers of different metals. The patent discloses that, for example, a catalyst could be manufactured by electroplating a thin film of platinum or gold onto finely divided conductive particles such as carbon.
U.S. Pat. No. 3,577,324 also discloses a process for coating particles with metals. No specific chemistry is claimed but rather an apparatus and accompanying process that allows particles as small as 10 microns to be uniformly coated without an accumulation of gas generated from the plating reaction on the surface to interfere with the deposition of particles.
Thus, none of the foregoing art has disclosed a process for the reductive deposition of heteropolyoxoanion complexes onto supports wherein the reduced, deposited materials are not reduced to their metallic state.